1. Field of the Invention
The present invention relates generally to battery powered vehicles for human transportation and, particularly, to hydraulic drive systems for wheelchairs.
2. Description of the Prior Art
Battery powered wheelchairs have enhanced the quality of life of persons of restricted mobility. Typically, such a vehicle is driven by a powered DC electric motor whose output is mechanically connected to the rear, or drive, wheels of the wheelchair. Batteries for powering the motor are stowed beneath the seat of the user while movement of the wheelchair is controlled by a joystick, or similar control device, mounted within the reach of the user, usually on or near an armrest. The movement of each drive wheel is independently controlled resulting in desired forward, reverse and/or turning motions of the wheelchair.
The running or operating time of battery powered wheelchairs is limited by the life of the batteries. Once a battery is drained of its useful voltage, it must be recharged, in known manner.
Other disadvantages of known electrically driven wheelchairs are their lack of smoothness in operation (particularly when the chair is being closely positioned relative to an article of furniture) and their inability to produce a significantly greater torque as the load increases. Hydraulic systems address both of these issues in that hydraulic controls provide a variable control function allowing a smoothness in operation, as during positioning, for example, while increasing loads will result, in a properly designed hydraulic system, in an increasing torque. However, prior hydraulic systems that are otherwise suitable to power a wheelchair do not address the question of power consumption. Also, such systems compound that problem through energy losses that must be dissipated as heat. The latter results from flow through portions of the system that is not required to meet an existing load condition.